cin.getline() isn't working with switch statement

Question

I've been trying to work on a program for class but for some reason when i input the number one for the program it goes to case 1 but then it doesn't let me enter a string and goes straight back to the menu. Ex) enter in 1 the result is: Enter a string: 01. Adds number but ignores anything that not a number. If I use a regular cin statement the code will execute perfectly fine. I don't understand why it is doing this. Can somebody help?

#include <iostream>
#include <cctype>

using namespace std;

void firstChoice(char []);

int main()
{
    int choice;
    int answer;
    const int SIZE = 100;
    char line[SIZE];


do
{
    cout << "1. Adds numbers but ignores anything thats not a number." << endl;
    cout << "2. Count the number of consonants in a string." << endl;
    cout << "3. Counts the vowels in a string." << endl;
    cout << "4. Counts whitespace characters in a string." << endl;
    cout << "Enter a number to access that program or 0 to end it: ";
    cin >> choice;

    switch(choice)
    {
        case 1:
            cout << "\nEnter a string: ";
            cin.getline(line, SIZE);
            firstChoice(line);
            break;

        case 2:
            cout << "Enter a string: ";
            cin.getline(line, SIZE);
            break;

        case 3:
            cout << "Enter a string: ";
            cin.getline(line, SIZE);
            break;

        case 4:
            cout << "Enter a string: ";
            cin.getline(line, SIZE);
            break;
    }
}
while(choice != 0);
return 0;
}

void firstChoice(char line[])
{
int size2 = 0;
int sum = 0;

while(line[size2] != '\0')
{
    if(isalpha(line[size2]))
    {
        line[size2] = 0;
    }
    sum += line[size2];

    size2++;
}
cout << sum;
}

Answer 1

After this statement

cin >> choice;

use

#include <limits>

//...

cin >> choice;
std::cin.ignore( std::numeric_limits<std::streamsize>::max(), '\n' );